Modern hoists utilize on-off switches for motor control, and as the payload reaches a desired position the switch is actuated on and off in quick sequence to gradually jockey the payload into position. Because of the high current loads during motor starting, thermal circuit protection devices must be sized for two to four times the steady state motor load. Thus, the motor circuits must be sized to accommodate current levels several times greater than operating levels.
Additionally, prudent design dictates that the mechanical components must be sized to withstand the large motor torque produced at the high current level attainable before circuit interruption by the thermal protection device.
One approach to solving the problem has been to employ time-delay circuit breakers or fuses that allow high current levels to flow for a period only slightly longer than that required for motor starting before circuit interruption. However, many motor starts in a short time period, such as when jockeying a payload, can cause a thermal build up sufficient to trip the thermal protection device. In addition, this jockeying of the payload imposes heavy loads on the mechanical components of the hoist as well as the payload itself.
Mechanical protection devices such as slipping clutches or shear pins are not usually satisfactory for use in hoists, since such devices cause impaired or lost control of the payload. Such impaired control may allow rapid dropping of the payload which would be extremely dangerous to nearby personnel and the payload as well.
Thus, difficulties have continued to exist in providing efficient and safe control devices for hoists and other similarly loaded mechanisms.